With the enormous expansion of activity in Semiconductor Technology over the last decade or so has come the need for chemicals of hitherto unheard of purity. We see reference to materials which typically have purity levels measured in fractions of a part per million being used routinely in wet-way chemical processes on the silicon wafers. These materials are used in aqueous solutions, which in turn places a very considerable emphasis on the purity of the solvent, water. The present of traces of inorganic contaminants in both the chemicals being used and in the water can lead to the irreversible modification of the semiconductor materials during processing.
The normal technique to asses the purity of water is to measure the concentration of the contaminants in their ionised state using standard conductivity methods. This method, while being most satisfactory at normal impurity levels such as in tap water, distilled water, and the like, becomes extremely difficult when the water approaches the ultimate in chemical purity with resistivities of 10's of M.OMEGA.cm, the level required in the semiconductor industry.
In the course of our research we have found that a tribo-electric e.m.f. which is generated due to friction in flowing water particularly when it is transported through insulating plastic pipes, can be used to measure water purity. In fact, this e.m.f. can cause damage to highly reflecting surfaces in water cooled pumping cavities in the solid laser. According to this invention use is made of this tribo-electric effect to measure the resistivity of the water, a function inversely of the ionic contamination and hence its purity, in a very simple measuring cell.
According to this invention the resistivity of water is measured by flowing water through a pipe of insulating material having linearly spaced electrodes, and measuring the tribo-electric e.m.f. generated by the flowing water, and calculating the resistivity of the water from this measurement.
In specific applications, such as the semiconductor technology referred to above, where generally the pipes are looped and the two ends are at the same potential, the invention is modified by having electrodes arranged to provide differing characteristics from adjacent sections of the cell so formed. This can be achieved by using materials of different characteristic or by having differing diameters in the cell to give a different velocity and hence achieve a different tribo-electric e.m.f.
While the invention has been based on the semiconductor industry, application of this effect could be widespread in many chemical plants and other industries.
Thus the invention can be applied as a method for measuring the purity of water which comprises flowing the water through a cell and measuring the tribo-electric e.m.f. generated by the flowing water, and calculating the resistivity of the water, the measuring of a tribo-electric e.m.f. being effected by flowing the water through a cell comprising a pipe of insulating material arranged between electrodes to which the measuring device is attached.
In the case where the two ends of the pipe are at a common potential, two dissimilar cell sections are preferably used which vary by wall characteristic, such as by using for instance a material known under the trade mark "Delrin" for one section of the cell and material known under the trade mark "Perspex" for the other section of the cell, arranged coextensive the one with the other with an end electrode at each end of the cell so formed and a further electrode between the two sections.
Instead of using two sections of the cell of dissimilar material the two sections can be of similar material but vary from each other by the internal diameter of the pipe sections.